THE SENSE OF SIGHT 



697 



axons of nerve-cells located in the superior cervical ganglion. They reach 

 the iris by way of the cervical sympathetic, the ophthalmic division of the 

 fifth, and the long ciliary nerve. Stimulation of these nerves is followed by 

 contraction of the dilatator and an increase in the size of the pupil. Both 

 the ciliary and superior cervical ganglia are in relation with pre-ganglionic 

 fibers coming from the central nerve system (see page 666). 



The Ciliary Muscle. The ciliary muscle is a gray circular band about two 

 millimeters in width, consisting of non-striated muscle-fibers. The majority 

 of these fibers pursue a radial or meridional direction. Taking their origin 

 from the junction of the sclera, cornea, and iris, they pass backward to be in- 

 serted into the chorioid coat opposite the ciliary processes. The inner por- 

 tion of the muscle is interrupted by bundles of fibers which pursue a circular 

 direction (Fig. 285). They collectively constitute the annular or ring 

 muscle of Mliller. The ciliary muscle in common with the circular fibers 

 of the iris receives its nerve supply direct from the nerve-cells in the ciliary 

 ganglion. Contraction of the 

 ciliary muscle tenses the cho- 

 rioid coat, and for this reason 

 it is frequently termed the ten- 

 sor chorioidea. 



The Retina. The retina 

 is the internal coat of the eye, 

 extending forward almost to 

 the ciliary processes, where it 

 terminates in an indented bor- 

 der, known as the ora serrata. 

 In the living condition it is 

 clear, transparent and pink 

 in color. After death it be- 

 comes opaque. The retina is 

 abundantly supplied with 

 blood-vessels, derived from 

 the arteria centralis retina, a 

 branch of the ophthalmic, 

 which pierces the optic nerve 

 near the sclera, runs forward 

 in its center, to the retina, in 

 which its terminal branches are distributed. The veins arising from the 

 capillary plexus leave the retina by the same route. 



In the posterior portion of the retina, at a point corresponding with the 

 axis of vision, there is a small oval area about 2 mm. in its transverse and 

 about 0.8 mm. in its vertical diameter. From the fact that it presents a 

 yellow appearance, it is known as the macula lutea. This area presents in 

 its center a depression with sloping sides, known as the fovea centralis. 

 About 3.5 mm. to the nasal side of the macula is the point of entrance of the 

 optic nerve. 



The retina is remarkably complex in structure, presenting an appearance, 

 when viewed microscopically, something like that represented in Fig. 288, 

 indicating that it is composed of different cellular elements arranged in 

 layers. These have been named, from behind forward, as follows: 



Pigment-layer (not shown). 



2. Layer of rods and cones. 



External limiting membrane. 



4. Outer nuclear layer. 



Outer molecular layer. 

 Inner nuclear layer. 



7. Inner molecular layer. 



8. Layer of ganglion cells. 



9. Layer of nerve-fibers. 



FIG. 285. VERTICAL SECTION OF HUMAN RETINA. 

 (Schaper.) 



